feat(pareto): add non_dominated_sort

Deb's fast non-dominated sort: returns Vec<Vec<usize>> of front indices
into the input population, with fronts[0] being the non-dominated set.
O(N²·M) is acceptable for v1 (spec §9.5).

Tests cover: small known population produces expected fronts; equal
candidates land on the same front; an empty population yields no
fronts.

src/pareto/mod.rs

@@ -2,6 +2,8 @@
 
 pub mod dominance;
 pub mod front;
+pub mod sort;
 
 pub use dominance::*;
 pub use front::*;
+pub use sort::*;